An optical information recording medium for recording/reproducing the information by irradiating a recording film with a convergent beam is conceived as a large capacity storage medium. In particular, an magneto-optical disk which uses a rare-earth metal--transition metal amorphous alloy for a recording layer is utilized as an erasable large capacity recording medium. The recording films of the rare-earth metal--transition metal amorphous alloy has a large perpendicular magnetic anisotropy. Magnetization is effected upwards or downwards on the film plane. When irradiating this film surface with a linearly polarized beam, a polarizing plane of the reflected light rotates through +.theta.k or -.theta.k depending on an orientation of magnetization. This phenomenon is known as Kerr effect. The information is reproduced by detecting a rotation of this polarizing plane. In a state where a magnetic field smaller than a coercive force at a normal temperature is applied, a temperature is increased in the vicinity of a Curie temperature enough to reduce the coercive force by irradiating the perpendicular magnetizing film with a convergent beam of sufficient power to increase the temperature and reduce the coercive force. Then, the magnetization takes place towards the applied magnetic field. Based on this principle, a write of a micro magnetic domain is performed, thus recording the information. A spot of the convergent beam can be set to 1 micrometer or under. Hence, the magneto-optical disk makes a high density storage possible. Problems to be solved by the Invention:
However, a much larger capacity storage medium is needed for effecting digital recording of image information, especially of a motion picture. The recording density is further increased to respond to a demand for decreasing a medium size. In the case of the magneto-optical disk, the recording density is determined by a spot diameter of the convergent beam. If a laser beam is employed as a source of the convergent beam, the spot diameter can be contracted down to a diffraction limit and is determined by a wavelength of the laser beam and by NA of a lens for convergence. To reduce the spot diameter, a value of NA may be increased. To secure a constant working distance, a lens diameter increases. Hence, there is a limit in terms of utilization. An as-employed lens diameter is 0.50-0.55, and the upper limit thereof is, it can be said, 0.65-0.70. When shortening the wavelength of the laser beam, the spot diameter is reduced. It is required that a semiconductor laser be used as a laser beam source to reduce a size of an optical head. Shortening of the wavelength is not, however, easily attained. Particularly, a short wavelength laser having a high and a long life is difficult of utilization thereof. An objective for the time being is to decrease the as-employed wavelength of 780 nm down to 670 nm.
It is very difficult to further increase the capacity of the optomagnetic disk on the line of extension of the prior art. Accordingly, the present invention aims at making multi-layer recording possible to increase the capacity of the magneto-optical disk without waiting for shortening of the wavelength of the laser and adoption of larger NA.